Bone lengthening through progressive distraction of the callus that forms naturally when a bone is fractured before it calcifies was initially done using external fixators, for example as made popular by Ilizarov.
To try to correct the drawbacks of external fixators, such as infections or annoyances in daily life, completely implanted lengthening nails were proposed: Albizzia nails, through which the lengthening is caused by a deliberate rotation of the patient's leg, Bliskunov nails, Baumgart and Betz nails, or the ISKD (Intramedullary Skeletal Kinetic Distractor) nail by Cole.
The speed of lengthening of the bone, on the order of one millimeter per day, is adjusted according to clinical observations.
Bone transport allows for reconstructing, for example, part of the diaphysis of a bone removed following a trauma, an infection, or a tumor, by gradually stretching the callus that forms between a preserved bone portion and a bone slice that has been detached from that portion through osteotomy, until the slice abuts against the second preserved bone portion.
Documents WO 02/071962 and WO 95/24870 present examples of bone transport nails. These two devices operate through pawl systems, which are bulky, comprise a large number of parts, and are difficult to miniaturize. The movements necessary to elongate them are frequently painful.
Distraction using intracorporeal means for the surgical treatment of deformities of the spine have also been proposed, in particular to treat progressive scoliosis in children. A functional presentation of the surgical treatment instrumentations for deformities of the spine is provided by Foster et al (The Spine Journal, pp. 652-694, 2005).
In children, it is necessary not only to correct the deformity as well as possible, but also to maintain the obtained correction all throughout growth while limiting that growth as little as possible. This requires that the geometry of the implanted distractor evolve over time. To that end, most known distractors require repeated surgeries, with the associated difficulties, costs, and risks, in particular those of infection. This is the case of the devices described in documents WO 2006/010844, WO 2007/051924, FR 2900563, FR 2843538, FR 2891727, FR 2794357 and FR 2892617. This is also the case for the VEPTR® (Vertical Expandable Prosthetic Titanium Rib). In order to try to limit the number of surgical operations, several intracorporeal devices that may be elongated without reoperation have been proposed. For example, the vertebral distractor described in documents WO 01/78614 comprises a magnet steering the rotation of the wheel meshing with two diametrically opposite wheels and each provided with a tapping complementary to the thread of two elongation rods.
The use of gears and threads on the elongation rods necessitates that the device be sealed against the surrounding biological materials, which is technically very delicate and limits sterilization possibilities. Furthermore, the presence of a thread on the elongation rods, which are very stressed in fatigue, limits the lifetime of the implant or requires that it be overdimensioned, which then makes it too voluminous to be reasonably implanted in a child.
Document WO 2007/144489, in the applicant's name, describes an intracorporeal elongation device which may be used as a bone lengthening nail, bone transport nail, spine distraction rod, or growth prosthesis. The device described in document WO 2007/144489 comprises:                a first elongated part,        a second part telescopically mounted relative to the first portion,        first means for connecting to the body, for example by screwing, at a first end of the first part,        second means for connecting to the body, for example by screwing, at a first end of the second part,        a rod comprising at least one thread whereof the rotation drives the movement of the second part relative to the first part,        means for controlling the rotation of the rod, for example a permanent magnet, said rod being mounted between two ends that come closer together when the device elongates.        
In the preferred embodiment described in this prior document by the applicant, the rod comprises, at a first end portion, a housing for a permanent magnet. A support tab is secured, for example by welding, to the second end portion of the rod. The support tab is secured to the first tubular part of the device, for example by welding, and is guided in longitudinal sliding relative to the second part of the device. Between the housing of the permanent magnet and the support tab, the threaded rod is screwed into a tapping of the second part of the device. A loaded rod length is thus defined between the tapping of the second part and the support tab rigidly connected to the first part of the device.
This prior device has many advantages. In particular, it is easy and inexpensive to manufacture in light of the function performed, has sufficient power for its function in a reduced volume, does not require particular sealing, can be controlled manually and painlessly using a simple external permanent magnet, in particular at home, by the patient or a layperson assisting the patient. Furthermore, the length of the loaded rod decreases when the device elongates, and the loaded rod length works in traction when the device elongates, which prevents any possibility of buckling.
The device described in document WO 2007/144489 does, however, have, for the desired power and elongation potential, a rigid straight length that makes it impossible to place in a location that is completely curved (such as applications to the spine and the thorax, for example) or a location where space is lacking lengthwise in the direction of movement that one wishes to gradually perform (such as for significant bone transports or elongations, or those of the jaw, for example). Furthermore, this device is capable of exerting a distraction or compression, but not both interchangeably, because it disassembles if a force is applied in the direction opposite that of the force it is capable of producing. The invention in particular aims to do away with these limitations while preserving the advantages of the device described in document WO 2007/144489 by proposing a new intracorporeal screw elongation device, with an adjustable length, in particular but not exclusively for an application to spine distraction or compression or bone transport.